1. Field of the Invention
The present invention relates generally to integrated circuits, and more particularly to cost effective and compact circuitry for enhancing power transistors.
2. Description of the Related Art
There are many computer and electronics applications in which power field effect transistors are being used. A common application is to use power field effect transistors to control the turn-on or turn-off of a load circuit. As is well known in the art of microelectronics, a load circuit (e.g., a circuit, a motor, an inductor, etc.) can be virtually any circuit that needs to be driven by some voltage source or current source. For example, in any given microelectronics application, circuit elements are driven by voltages that range between a rail voltage (Vcc) and a voltage (Vss), which may be a negative voltage or ground. However, enhancement mode, N-type power field effect transistors will generally require that the gate-to-source voltage be more positive that the rail voltage (Vcc) to achieve full enhancement. To accomplish this, designers typically use charge pump circuitry that, although simple in design, may be expensive and require a substantial amount of semiconductor die area to complete the circuit layout.
To elaborate further, FIG. 1 shows a prior art charge pump converter 10 that is used to provide an enhanced voltage (ie., Vcc+V.sub.1) to the gate of an N-type power field effect transistor 12. Typically, the charge pump converter is required to produce a certain step-up ratio. Commonly, the charge pump converter circuitry implements boost stages to achieve the desired step-up ratio. Unfortunately, to produce greater step-up ratios, more boost stages will be required. That is, more boost stages and semiconductor die area is required to layout a quadrupler than is required to layout a doubler. As an example, to generate a bias gate voltage that is +8V larger than a +3V rail voltage supply, a quadrupler circuit would be required (i.e., 4*3V=12V). However, to generate a bias gate voltage that is +8V greater than a +12V rail voltage supply, only a doubler circuit would be required (i.e., 2*12=24).
In some applications, designers use specialized integrated circuit (IC) chips whose, sole purpose is to generate an increased voltage to ensure that a power field effect transistor becomes fully enhanced when required. For example, Maxim Integrated Products of Sunnyvale, California, stocks an individually packaged IC chip that outputs a regulated output voltage that is +11V greater that the rail voltage supply (Vcc). This IC chip is the "MAX620/MAX621" Quad, High-Side MOSFET Driver. The technical details of this IC chip, which can be obtained from Maxim Integrated Products, is hereby incorporated by reference. Typical applications for this chip include, portable computer battery load management, high-side power N-channel MOSFET switching, low-side switching from low supply voltages, quad-latching level transistors, and stepper motor drivers. Although the uses of such an IC chip are many, this IC chip must be routed to other applications over a PC board, and if it is integrated onto a chip along with other circuitry, the circuitry needed to route and complete the charge pump circuit components can be quite substantial and expensive.
In view of the foregoing, there is a need for simple and compact circuitry that may be used to generate a gate bias voltage that is greater than a standard rail voltage, without the need for large and possibly expensive space consuming charge pump circuitry.